This invention relates to a method for recovering silver from silver exchanged zeolite. More specifically, this invention relates to a method for the recovery and purification of silver from silver exchanged zeolite which may also contain some radioactivity.
The removal of air-borne radioactive iodines from the atmosphere to prevent surface contamination and possible human ingestion is an important aspect of nuclear safety. These radioactive iodines, which include inorganic species such as elemental iodine, and organic species, such as methyl iodine, may be present in reactor containment vessels from normal reactor operations and, in particular, in the case of fuel element cladding failure and are present in dissolver off-gases from nuclear fuel reprocessing plants. These radioactive iodines are formed in the nuclear fuel by the fissioning of the fuel material.
One method for removing these iodines from the atmosphere is the use of activated charcoal filter beds. However, charcoal will ignite at temperatures above about 300.degree. C. and becomes inefficient when subjected to a high humidity environment.
An effective substitute for charcoal for the removal of both organic and inorganic iodine species from the atmosphere is silver exchanged zeolite. Silver exchanged zeolite is a synthetic zeolite in which the sodium ions have been replaced by silver ions and is prepared by passing an aqueous solution of silver nitrate through a column of synthetic zeolite material which has been suspended in water in the manner described in U.S. Pat. No. 3,658,467 which issued April 25, 1972, is assigned to the common assignee, and is incorporated herein by reference. The silver exchanged zeolite is more effective for removing iodines from the atmosphere than charcoal and will retain radioactive iodines at temperatures above 500.degree. C. Furthermore, the effectiveness of silver exchanged zeolite is not impaired by a high humidity environment and most importantly it is noncombustible.
Silver exchanged zeolite which is loaded with fission product iodine can be reactivated and the radioactive iodine permanently stored by passing a stream of dry hydrogen gas through the bed to remove the iodine as hydrogen iodide which is then sorbed on lead exchanged zeolite which is then converted into a waste form suitable for permanent storage. This is described in U.S. Pat. No. 4,088,737 which issued May 9, 1978 and is assigned to the common assignee.
The cost of silver exchanged zeolite is high because of the silver and the material is subject to poisoning by hydrocarbons which occasionally make their way into reactor and fuel reprocessing plant environments. No successful process has been developed to reactivate poisoned silver zeolite filter beds, so that at present the only solution is to replace the filters with fresh materials.
Due to the necessity of replacing silver exchanged zeolite filters because of poisoning, many thousand pounds of silver exchanged zeolite, some of it slightly radioactive, is presently in storage. Because of the value of the silver in this zeolite, which may vary from 30 to 40 weight percent (w/o), it would be desirable if this silver could be recovered and purified for either reuse or sale. Furthermore, environmental regulations prohibit disposal of the zeolite by burial because of the presence of a heavy metal, i.e. silver, in the zeolite.
Efforts so far to recover the silver from the zeolite have utilized isopropylamine. In this method, the silver zeolite is loaded into a column and contacted with the isopropylamine which washes the silver from the zeolite. However, the isopropylamine is able to remove only silver which is still in the ionic state. About 66% of the silver is recoverable by this method leaving the zeolite still containing about 10 w/o silver. Furthermore, isopropylamine is flammable and unpleasant to use.